Retrofitting Garage Floor Trench Drains

A trench drain collects water and routes it out and away from the garage.

Residential building codes specify the slope of a new garage floor to ensure that water drains out and away from the garage. If the garage sits at the bottom of an incline, or if you use water in the garage to wash automobiles regularly, the minimal slope may not be sufficient to remove all of the water. Alternately, if the garage is older, the floor may not comply with minimal slope specifications. By retrofitting the existing floor with a trench drain, the water will drain out without puddling in a corner or damaging the bottom of the garage framing and drywall.

1

Measure the garage floor where you want to install the trench and snap two chalk lines to represent the outside edges of the trench. Drain widths vary, but 5 to 6 inches is typical for a residential trench drain.

2

Snap two additional chalk lines 4 inches on the outside of the first set of lines. These are the cut lines. You must cut the trench wider than the actual drain width to secure the drain with concrete.

3

Cut along both outside cut lines with a concrete saw.

4

Break out the concrete between the two cut lines with a jackhammer.

5

Remove the concrete debris from the trench and dig the ground beneath to a depth of at least 3 inches deeper than the depth of the trench drain.

6

Fill the bottom of the trench with 2 inches of fill sand and level the sand.

7

Assemble the drain sections in the trench and secure it as recommended on the installation sheet. Some drains come with rods to drive in the ground with a mallet and others rest on the sand bed.

8

Adjust the top level of the trench drain. Standard trench drains are formed from heavy-duty polyethylene and feature a “lip” on both interior top edges. The lip holds an iron grate than cannot be higher than the level of the garage floor. Put the grate in place and use the flat edge of a board, or a short straightedge to ensure the grate is level, but that no areas are higher than the garage floor on either side.

9

Fill the sides of the trench with wet concrete. The best way to do this is to mix small amounts of premixed concrete with water according to package directions as you go. Use a small shovel or a concrete trowel to work the wet concrete into and around the trench, pushing and tamping it to remove air voids.

10

Screed the top of the wet concrete to be even with the level of the existing garage floor and the top of the trench drain. Let the concrete set for at least two weeks before driving a vehicle over it.

Things You Will Need

Tape measure

Chalk line

Concrete saw

Jackhammer

Shovel

Fill sand

Rebar (optional)

Mallet

Straight edge

Premix concrete

Water

Wheelbarrow

Concrete trowel

Tip

Trench drains are configurable. You can purchase corners and bends that fit around garage walls or in tight spots.

The drain should exit on one or both sides of the garage or driveway to allow water to run out.

Warning

Using a concrete saw and jackhammer requires physical strength and both create airborne dust and debris. Wear a respirator and goggles and follow the manufacturer’s safety precautions when using power tools.

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